Safety assembly for a syringe pre-filled with liquid, in particular a medicine

ABSTRACT

The syringe (S) comprises a body (K) forming a liquid reservoir, carrying a needle (A) for injecting the liquid, and having a plunger (P) movably mounted in the body (K). The body (K) is housed in a safety sheath ( 1 ) to be axially movable between an active position in which the needle (A) projects through a distal end of the sheath ( 1 ) and a protection position in which the needle (A) is retracted inside the sheath ( 1 ) and towards which position the body (K) is urged resiliently. The body (K) is prevented from moving relative to the sheath in the active position by locking means ( 6 ) that opposite the resilient force on the body, and that are releasable by means ( 8 ) which are activated when the plunger (P) is in an end-of-liquid-injection position.

BACKGROUND OF THE INVENTION

The present invention relates to a safety assembly for a syringeprefilled with injection liquid, in particular a medicine.

In the state of the art, a safety assembly is already known for aprefilled liquid injection syringe comprising a tubular body that formsa reservoir for the liquid, and that carries a needle for injecting theliquid and a plunger that is mounted in the body to be movable between aready position and an end-of-injection position.

Usually, the plunger of a syringe has a drive end outside the body ofthe syringe and an end inside the body of said syringe that carries apiston. The drive end of the plunger is also known as a thumb rest.

Certain medical substances such as vaccines, heparins and derivativesthereof, and other accurately-dosed medicines, cannot be stored incontainers or syringes made of plastics material because of the rate atwhich moisture is taken up, which would run the risk of compromising thecomposition.

Aware of these imperatives, laboratories are constrained to use onlycontainers made of glass which presents no porosity. Given infectiousvirus diseases for which there is at present no antidote, safety iscurrently sought for providing all health care personnel with protectionagainst any risk of possible contamination during professional accidentsdue to injections. This protection must come close to the 100% rate thatthe profession is calling for.

The problem is thus to fit a safety mechanism to syringes that arepreferably made of glass, in particular syringes having capacity in therange 0.1 milliliters (ml) to 2 ml, since any type of mechanism internalto such syringes is unsuitable.

In addition, the requirement for sterility excludes any internalhandling of a syringe once it has been filled.

Various solutions have been presented to laboratories and set aside forvarious reasons: increase in volume, modification to filling assemblylines, complex operations required of the doctor or the personnel usingthose proposals, pseudo-protection that is more harmful than useful.

It should be observed that numerous inventors can be criticized fortheir misleading use of language such as “a syringe having anautomatically withdrawable needle” which, very often, turns out to be nomore than a syringe which locks automatically, or a syringe havingelements that can be destroyed, or having a cap or sliding tube to hidethe needle after use.

In practice, a nurse holds a syringe between the index and middlefingers and presses with the thumb against the thumb rest. As a result,while the syringe is being held, the needle can be retracted only into ahollow plunger or piston, since the body of the syringe remainsstationary.

In other cases, a sheath that slides longitudinally on the outside ofthe syringe “covers” the needle and thus in fact amounts to “retracting”the needle by advancing the sheath either manually or automatically.

Numerous patents are based on that known configuration which has beenrejected by the medical profession because of the risk of the needlebeing pulled out or the patient suffering an incision as a result of theshock of the sheath striking the patient under the force of a springurging the sheath into the needle-retraction position.

SUMMARY OF THE INVENTION

An object of the invention is to remedy the above-described drawbackswhile satisfying the following criteria: good compactness, suitable forfitting to the end of a manufacturing line prior to packaging, safe foroperating with one hand, automatic, controlled independently of humanwill, protection very close to the desired 100%.

To this end, the invention provides a safety assembly of theabove-specified type, characterized in that it comprises:

a tubular sheath in which the body of the syringe is designed to behoused in axially displaceable manner between an active position inwhich the needle projects through a distal end of the sheath and aprotection position in which the needle is retracted inside the sheath;

resilient return means for urging the body towards its protectionposition; and

locking means for preventing the body from moving relative to the sheathin the active position by opposing the resilient force of the returnmeans, said locking means being releasable by means activated when theplunger is in its end-of-injection position.

According to other characteristics of this assembly:

the sheath carries external retention means for being held by thefingers of a user to inject the liquid by moving a drive end of theplunger axially towards the retention means;

the retention means of the sheath comprise two substantiallydiametrically opposite fins or lugs;

the retention means comprise a shoulder formed on the outside surface ofthe sheath;

the body of the syringe has a proximal end provided with a flange, andthe locking means comprise a seat against which the flange bears, whichseat is formed in a proximal end of the sheath, and also comprisesretractable means for clamping the flange against the seat, said meansbeing carried by said proximal end of the sheath;

the retractable clamping means comprise at least one retractable catch;

the means for resiliently urging the body of the syringe into theprotection position comprise a thrust spring designed to bear bothagainst the flange of the body of the syringe and also against aninternal bearing shoulder formed in the sheath between the bearingshoulder for the flange and the distal end of the sheath;

the means for releasing the locking means comprise a cap mounted toslide axially on the proximal end of the sheath between a ready positionand a position for retracting the clamping catch, the cap being designedto be secured to the plunger at least during a portion of thedisplacement stroke of the plunger from its ready position to itsend-of-injection position;

the cap has means for securing it to the plunger;

the cap and the proximal end of the sheath comprise complementary meansfor limiting the stroke of the cap in opposition to the resilient forceof the means for returning the body after the locking means have beenreleased;

the complementary means for limiting the stroke of the cap comprisecomplementary annular shoulders forming abutments that are provided onthe sheath and the cap;

the complementary means for limiting the stroke of the cap comprisecomplementary snap-fastening means carried by the cap and the sheath;

the sheath and the cap are generally in the form of bodies of revolutionand have complementary means for preventing relative rotation betweeneach other;

the complementary means for preventing relative rotation of the sheathand the cap comprise at least one longitudinal groove formed in the capand co-operating with a corresponding finger secured to the sheath;

the complementary means for preventing relative rotation of the sheathand the cap comprise at least one axial slot formed in the cap andco-operating with a fin;

the retractable clamping means comprise two arms hinged to the proximalend of the sheath (1) about a common transverse axis so as to formtongues for clamping the flange of the body of the syringe, each armhaving an end for clamping the flange against its bearing seat and anend for driving the arm;

the drive ends of the arms co-operate with the retention lugs in such amanner as to be displaced when the plunger reaches its end-of-injectionposition from a position of forced clamping of the flange to a positionof forced release of said flange, the passage from the forced clampingposition to the forced release position taking place by passing througha hard point; and

the user presses the fingers during injection of the liquid both againstthe retention lugs and the arms; and

the proximal end of the sheath is extended by two longitudinal tabs forco-operating by snap-fastening with the flange of the body of thesyringe so as to limit the stroke of said body after the locking meanshave been released.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the followingdescription given purely by way of example and made with reference tothe accompanying drawings, in which:

FIGS. 1 to 3 are axial section views of a safety assembly for a syringeand constituting a first embodiment of the invention and shownrespectively in three configurations of use;

FIGS. 4 and 5 are views of a safety assembly for a syringe andconstituting a second embodiment of the invention, shown in axialsection on two mutually perpendicular planes, the plunger of the syringebeing in its ready position;

FIG. 6 is a view similar to FIG. 5 in which the plunger of the syringein its end-of-injection position;

FIG. 7 is a view similar to FIG. 4 showing the position of the plungerof the syringe after the means for locking the body of the syringe hasbeen released;

FIGS. 8 to 10 are axial section views of a safety assembly for a syringeand constituting a third embodiment of the invention, shown in threeconfigurations of use, respectively;

FIG. 11 is an axial section view of the protective sheath of theinjection assembly shown in FIGS. 8 to 10; and

FIGS. 12 and 13 are views of the cap of the injection assembly shown inFIGS. 8 to 10, in axial section on two mutually perpendicular planes

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIGS. 1 to 3 show a safety assembly for a conventional syringe S forinjecting a liquid, in particular a medical liquid, and constituting afirst embodiment of the invention. This safety assembly is given overallreference E.

The syringe S is prefilled, ready for use, and comprises a tubular bodyK forming a reservoir for the liquid, carrying a needle A for injectingthe liquid, and having a plunger P mounted in the body K to slidebetween a ready position, as shown in FIG. 1, and anend-of-liquid-injection position as shown in FIG. 2.

The plunger P has a first end PE outside the body K and a second end PIinside the body K and carrying a conventional piston PO. The outside endPE forms an end for driving the plunger.

The assembly E comprises a tubular safety sheath in which the syringe isreceived, and in particular the body K. The sheath is a body ofrevolution and is constituted, for example, by a cylindrical tube 1 ofplastics material or any other material that is sufficiently transparentto enable the liquid contained in the body K of the syringe to be seentogether with any label for identifying the medicine. It will beobserved that the cylindrical tube 1 is stepped.

In the description below, an element is referred to as “proximal” or“distal” depending on whether it is axially close to or remote from theoutside end PE of the plunger.

The body K is held stationary relative to the tube 1 by locking meansfor preventing the syringe body from moving relative to the sheath ortube 1. The locking means comprise firstly a seat 4E against which theflange CL bears, the seat being formed by an internal shoulder definingthe second housing 4, and secondly retractable means for clamping flangeCL against the seat 4E and carried by the proximal end of the tube 1.

It will be observed that the body K of the syringe has a distal endcarrying the needle A and a proximal end fitted with a flange CL.

The inside surface of the tube 1 has three guides 2 in relief extendingin the longitudinal direction of the tube 1 and spaced apart from oneanother at 120° intervals, for the purpose of centering the body K ofthe syringe, reducing play, and improving the sliding of the body Kinside the tube 1, while serving to pass a conventional rubber cover C(shown in chain-dotted lines in FIG. 1) that protects the needle Abefore the syringe is used.

The proximal end of the tube 1 carries, going upwards: a firstcylindrical internal housing 3 extended by a second cylindrical internalhousing 4 of diameter greater than that of the first internal housing.

The body K is held stationary relative to the tube 1 by locking meanscomprising firstly a seat 4E against which the flange CL bears, the seatbeing formed by an internal shoulder defining the second housing 4, andsecondly retractable means for clamping the flange CL against the seat4E and carried by the proximal end of the tube 1.

In the example shown in FIGS. 1 to 3, the retractable clamping means areformed by two tabs 5, that prevent the syringe body from moving relativeto the sheath or tube 1, that are substantially diametrically oppositeand that are partially cut out from the wall of the tube 1 where itdefines the second housing 4. The tabs 5 are terminated by retractablecatches 6 for holding the flange CL of the syringe body pressed againstthe seat 4E. It will be observed that the tabs 5 are folded so as toextend substantially in a diameter longitudinal plane of the tube. In avariant, the retractable clamping means could comprise a single tab 5only.

The body K of the syringe is urged resiliently towards its protectionposition by a resilient return means, such as a thrust spring R placedin the first housing 3. This spring R bears firstly against an internalshoulder 3E defaming the first internal housing 3, and secondly againstthe flange CL of the body K of the syringe. The internal bearingshoulder 3E is disposed between the bearing seat 4E for the flange CLand the distal end of the tube 1.

In FIG. 1 which shows the safety assembly and the syringe ready for use,with the spring R compressed. The spring remains in this state until theliquid has finished being injected.

The catches 6 thus serve to prevent the body K moving relative to thetube 1 in the active position by opposing the resilient return force ofthe spring R.

It should be observed that the proximal end of the second internalhousing 4 is defined by an internal ring 7 integrally molded with thetube 1.

The means for locking the body K of the syringe relative to the tube 1can be released by means that are activated when the plunger P is in itsend-of-injection position.

In the example shown in FIGS. 1 to 3, the means (release means) forreleasing the locking means comprise a cylindrical cap 8, preferablymade of plastics material, mounted to slide axially in the secondhousing 4 of the tube 1 between a high, ready position as shown in FIG.1 and a position for retracting the catches 6, as shown in FIG. 2. Theplunger P passes through the cap 8 and extends via a hole 9 formed inthe proximal end of the cap 8.

The cap 8 also has snap-fastening means comprising internal catches 10that are spaced apart by 90°, and also has an external ring 11 that isintegrally molded with the distal end of the cap.

The internal ring 7 of the second housing 4 and the external ring 11 ofthe cap 8 define complementary annular shoulders forming abutments thatlimit the stroke of the cap 8 after the means for locking the body Khave been released, opposing the resilient return force of the spring R(see FIG. 3). The complementary rings 7 and 11 thus act as a returnlimiter for the cap 8.

Depending on the type of syringe in particular, it is preferable for thetube 1 and the cap 8 to have complementary means for preventing relativerotation between them. By way of example, these complementary means forpreventing rotation between the sheath or tube 1 and the cap 8 cancomprise at least one longitudinal groove 13 formed in the cap 8receiving a corresponding finger 13A that radially extends the internalring 7 of the second housing 4. The cap preferably has two diametricallyopposite grooves 13 as shown in FIGS. 1 to 3.

The syringe can be installed in the safety assembly E as follows.

Initially, the body K of the syringe is filled with the liquid that isto be injected. This liquid is retained in the body K by the piston PO.The plunger P is separate from the piston.

The body K of the prefilled syringe is placed in the tube 1 by snappingthe flange CL beneath the catches 6 after compressing the spring R inthe first housing 3.

Thereafter, the cap 8 is engaged in the second housing 4, e.g. beingpressed into said second housing 4 until the external ring 11 comes intocontact with the catches 6, as shown in FIG. 1.

Finally, the plunger P of the syringe is connected to the piston PO byinserting the distal end of the plunger into the cap 8 and the tube 1through the orifice 9 of the cap 8.

The safety assembly E is then ready for use in the configuration shownin FIG. 1, with the needle A of the syringe projecting through thedistal end of the tube 1.

In order to obtain an automatically-withdrawable needle and syringeeffect, it is essential for the safety assembly E to be taken hold ofvia the protection tube 1 so as to enable the syringe fitted with itsneedle to withdraw freely and properly in line as soon as injection hasterminated, i.e. without any risk for the patient. This provides anassembly E that is properly referred to as being of the type having anautomatically-withdrawable needle.

The tube 1 is taken hold of by external retention means carried by thetube 1, for being held by the fingers of a user in order to inject theliquid by moving the external end PE of the plunger axially towards theretention means.

In the example shown in FIGS. 1 to 3, the external retention means ofthe tube 1 comprise a shoulder 1E formed in the outside surface of thetube 1.

In order to inject the liquid, the user takes hold of the assembly Ebetween the index and middle fingers. The thumb exerts syringe pressureon the outside end PE of the plunger. The index and middle fingersretain the tube 1 by engaging the retention shoulder 1E.

The user pushes the plunger PE into the body of the syringe so as tobring the end PE of the plunger axially closer to the retention shoulder1E, and this continues until the end-of-injection position as shown inFIG. 2 is reached.

The displacement stroke of the plunger P from its ready position to itsend-of-injection position includes a final stroke during which theplunger P is engaged with the cap 8, preceded by an initial dead strokeof the plunger P relative to the cap 8 during which the cap 8 remainsstationary. During the engagement stroke of the plunger P with the cap8, the outside end PE of the plunger forming an engagement headco-operates with a setback defining the edge of the orifice 9 of thecap.

Because the cap 8 is entrained by the plunger P, the external ring 11 ofthe cap co-operates with the catches 6 so as to cause them to retract,as shown in FIG. 2. FIG. 2 also shows that the body K of the syringe hasbeen emptied of its content, the liquid having been injected into thebody of a patient by the user. The plunger P is at the end of itsstroke.

Because the catches 6 have been retracted, and when the thumb of theuser ceases to exert pressure on the external end PE of the plunger, thespring R relaxes, urging the empty syringe upwards (as shown in thefigures) so as to cause the flange CL of the body to snap-fasten betweenthe edge of the orifice 9 in the cap and the catches 10, as shown inFIG. 3. The cap 8 also moves under drive of the return force of thespring R towards the position shown in FIG. 3 in which the rings 7 and11 co-operate with each other to limit the stroke of the cap 8 and thusthe withdrawal stroke of the syringe. In the configuration shown in FIG.3, the needle A is retracted into the tube 1, thus providing 100%safety.

FIGS. 4 to 7 show a safety assembly E constituting a second embodimentof the invention. In these figures, elements that are analogous to thoseof the preceding Figures are designated by references that areidentical.

In this case, the tube 1 does not have the second internal housing ofthe first embodiment. Furthermore, the external retention means of thetube 1 are constituted by two diametrically opposite fins or lugs 15extending substantially transversely to the axis of the tube andextended in the longitudinal direction (parallel to the axis of the tube1) by two tabs 16 themselves terminated by oppositely-directed catches17, 18. The lugs 15 are preferably integrally molded with the tube 1.

Two pivots 19 projecting from the outside surface of the proximal end ofthe tube 1 are also integrally molded with the tube 1 at 90° from thetwo retention lugs 15. The pivots 19 define an axis extendingsubstantially transversely relative to the tube 1.

The two pivots 19 carry two respective identical arms 20 and 21 in theform of very widely-open U-shapes. These arms 20 and 21 are inverted bytheir positioning at 180°. Each arm 20, 21 has a pivot hole 22 forengaging on the corresponding pivot 19. Each arm 20, 21 has a shortportion and a long portion on opposite sides of the corresponding pivot22. The short portion is terminated by an end that is bent substantiallythrough a right angle, carrying a catch 23. The long portion is likewiseterminated by an end bent substantially through a right angle, includingan internal projection 24 (see FIGS. 5 and 6).

Both arms 20 and 21 are hinged about a common axis, and together theyform a retractable clamp for holding the flange CL of the body of thesyringe pressed against the seat 4E defined by the edge of the proximalend of the tube 1.

The bent ends of the short and long portions of the arms 20 and 21respectively form ends for clamping the flange CL against the seat 4Eand ends for driving the arms 20 and 21.

When the syringe is put into place inside the tube 1, the catches 23retain the flange CL of the syringe in the tube 1 while compressing thespring R in the housing 3.

The arms 20 and 21 are placed in a position in which they apply forcedclamping on the flange CL because their projections 24 are positionedbeneath the retention lugs 15. This position is symmetrical because ofthe way the arms 20 and 21 are placed at 180° relative to each other(see FIG. 5).

To inject liquid into the body of a patient, the user applies twofingers (generally the index and the middle finger) simultaneouslyagainst the lugs 15 and the arms 20, 21.

During injection, the drive ends of the arms 20 and 21 co-operate withthe lugs 15 so as to be moved when the plunger reaches itsend-of-injection position from the position in which they apply forcedclamping on the flange CL as shown in FIG. 5 to a position in which theyare forced to release said flange CL, as shown in FIG. 6. The passagefrom the forced clamping position to the forced release position of theflange takes place by passing through a hard point and by elasticdeformation of the arms 20 and 21. At the end of injection, the userpresses hard against the outside end CE of the plunger so as to expelall of the liquid contained in the syringe. This action causes theprojections 24 to escape from the lugs 15, thereby moving the lugs 23away from the flange CL and releasing the spring R.

By relaxing, the spring R pushes the syringe between the tabs 16. Theflange CL on the body of the syringe then co-operates with the two tabs16 that extend the proximal end of the tube 1 by snap-fasteningtherewith. The locking of the flange CL between the catches 17 and 18limits the stroke K of the syringe after the spring R has relaxed, ascan be seen in FIG. 7. In FIG. 7, the needle of the syringe is retractedinside the tube 1 after being automatically withdrawn so as to beprotected inside the tube 1 from any possible contact.

It should be observed that syringes prefilled with vaccine must beconserved and transported at low temperature. For practical and economicreasons, it is desirable to reduce as much as possible the overalldimensions of the needle protection means, since any increase in thevolume of these protection means has repercussions on the storage andtransport of injection assemblies. In addition, the injection of certainvaccines is preceded by a “vein” test which consists in jabbing theneedle into the body of a patient, holding the syringe firmly, andpulling on the outside end of the plunger so as to establish suction. Ifa small trace of blood appears in the portion of the syringe where theneedle is fixed, then injection must not be performed. The user mustthen jab the patient again a few centimeters away from the precedingjab. Under no circumstances must the vaccine be injected into the bloodcirculation network.

FIGS. 8 to 13 show a safety assembly E constituting a third embodimentof the invention. This embodiment makes it possible in particular to usea syringe that contains a vaccine.

In FIGS. 8 to 13, elements that are analogous to those of the precedingfigures, are designated by references that are identical.

In FIGS. 8 to 10, there can be seen elements that are analogous to theelements of the preceding embodiments, namely a tube 1 containing thehousing 3 in which the spring R is located. The proximal end of the tube1 is extended by two catches 33 and 34 which are preferablydiametrically opposite. In a variant, the number of catches 33, 34 couldbe less than or greater than two. The catches 33, 34 constitute lockingmeans having the function of preventing the body K of the syringe frommoving relative to the tube by clamping the flange CL of said bodybetween the catches 33, 34 and the bearing seat 4E. The function of thecatches 33, 34 is thus analogous to the function of the catches 6 in thefirst embodiment of the invention.

Two retention fins 35, 36 analogous to the lugs 15 of the secondembodiment are integrally mole de with the proximal end of the tube 1and are disposed at 180° to each other. The tube 1 is shown from theassembly E in FIG. 11. The cap 8 (see in particular FIGS. 12 and 13) isprovided in this case with two substantially diametrically oppositelongitudinally-extending tongues 38 and 39 which extend its distal end.The proximal end of the cap 8 is pierced by a hole 40. The cap 8 alsohas complementary means for preventing relative rotation between thesheath or tube 1 and the cap 8 comprising two vertical longitudinalslots 41 and 42 that are offset by 90° from the tongues 38, 39.

The slots 41, 42 co-operate with the fins 15 by receiving them in such amanner as to form complementary means for preventing the cap 8 fromrotating relative to the tube 1.

The catches 33, 34 are retractable so as to release the flange CL byco-operating with ramps 43, 44 defining the proximal ends of the slots41, 42.

The hole 40 in the cap serves to receive the plunger in the injectionassembly. The plunger P is secured to the cap 8 by conventional means(not shown).

FIG. 8 shows the safety assembly E in a configuration ready for use, theneedle of the syringe projecting through the distal end of the tube 1.Starting from this configuration, the user performs an injection, inparticular an intramuscular or a subcutaneous injection, by holding thetube 1 between the index and middle fingers, and using the fins 15 whilepressing the thumb against the outside end PE of the plunger. Since thecap 8 is secured to the plunger P, the cap is pushed under pressure fromthe thumb in the same direction as the plunger P until it reaches theposition shown in FIG. 9.

When the plunger P reaches its end-of-injection position, as shown inFIG. 9, the cap 8 moves away the catches 33, 34, by means of the ramps43, 44 of the cap 8 co-operating with the catches 33, 34, therebyreleasing the spring R.

The spring then urges the empty syringe body towards the proximal end ofthe cap 8, as shown in FIG. 10.

After the catches 33, 34 have been released, the stroke of the cap 8 islimited by snap-fastening means opposing the resilient return force ofthe spring R. These snap-fastening means comprise locking studs 45, 46formed on the free ends of the tongues 38, 39 co-operating withdiametrically opposite holes 47 formed in the proximal end of the tube 1by engaging therein (see FIG. 11).

FIG. 10 shows the configuration of the safety assembly E after use, andit can be seen that the needle is retracted inside the tube 1.

In the third embodiment of the invention, the syringe and the needle areindeed automatically-withdrawable, with the tube 1 constituting a stableportion that acts as a fixed bearing portion.

It should be observed that the fins 35, 36 of the tube 1 providestability on a non-plane surface.

The embodiments described above enable the needle to be retractedaccurately in line relative to its insertion, while also performing ananti-accident function for health care personnel.

Accidental contagion becomes impossible since the distal end of the tube1 is of small diameter, e.g. 9 millimeters (mm) thus preventing anyaccidental insertion of a user's finger. The safety of the assembly E isthus practically 100%, and, in accordance with the object of theinvention, the safety feature is triggered (the needle is retracted)automatically and independently of human will.

Furthermore, the needle is retracted using one hand only and withoutrequiring any special action to be taken, complying with medicalregulations and also with the normal practices of health care personnel.

The invention also enables health care personnel and other people calledon to handle, transport, or destroy used needles to avoid any risk ofaccidents.

1. A safety assembly for a prefilled syringe for injecting liquid, thesyringe comprising a tubular body forming a reservoir for the liquid,carrying a needle for injecting the liquid, and having a plunger mountedin the body to be movable between a ready position and anend-of-injection position, the body of the syringe having a proximal endprovided with a flange, the assembly further comprising: a tubularsheath in which the body of the syringe is designed to be housed inaxially displaceable manner between an active position in which theneedle projects through a distal end of the sheath and a protectionposition in which the needle is retracted inside the sheath; resilientreturn means for urging the body towards its protection position;locking means for preventing the body from moving relative to the sheathin the active position by opposing the resilient force of the returnmeans; release means for releasing said locking means when the plungeris in its end-of-injection position; the locking means comprising twodiametrically opposite lugs formed in a wall of the tubular sheath, eachlug having a free end provided with a retractable catch; a cap mountedto slide axially on the sheath between two positions and having aproximal end with a hole for receiving the plunger; snap-fastening meanscomprising an internal catch for securing the cap to the tubular body,so that due to retraction of the retractable catches, the cap is movedby resilient return means; and two complementary shoulders acting asabutments, one on the cap, the other one on the tubular sheath, to limitthe stroke of the cap once the locking means have been released, inopposition to the resilient force of the return means.
 2. The assemblyaccording to claim 1, wherein the sheath carries external retentionmeans for being held by the fingers of a user to inject the liquid bymoving a drive end of the plunger axially towards the retention means.3. The assembly according to claim 2, wherein the retention means of thesheath comprise two substantially diametrically opposite fins or lugs.4. The assembly according to claim 2, wherein the retention meanscomprise a shoulder formed on the outside surface of the sheath.
 5. Theassembly according to claim 1, wherein the means for resiliently urgingthe body of the syringe into the protection position comprise a thrustspring designed to bear against an internal bearing shoulder formed inthe sheath.
 6. The assembly according to claim 1, wherein the sheath andthe cap are generally in the form of bodies of revolution and havecomplementary means for preventing relative rotation between each other.7. The assembly according to claim 6, wherein the complementary meansfor preventing relative rotation of the sheath and the cap comprise atleast one longitudinal groove formed in the cap and co-operating with acorresponding finger secured to the sheath.
 8. The assembly according toclaim 6, wherein the complementary means for preventing relativerotation of the sheath and the cap comprise at least one axial slotformed in the cap and co-operating with a fin.
 9. The safety assemblyaccording to claim 1 further comprising an injection device comprising aprefilled syringe for injecting liquid, the syringe comprising a tubularbody forming a reservoir for the liquid, carrying a needle for injectingthe liquid, and having a plunger mounted in the body to be movablebetween a ready position and an end-of-injection position.
 10. Thesafety assembly according to claim 2 further comprising an injectiondevice comprising a prefilled syringe for injecting liquid, the syringecomprising a tubular body forming a reservoir for the liquid, carrying aneedle for injecting the liquid, and having a plunger mounted in thebody to be movable between a ready position and an end-of-injectionposition.
 11. The safety assembly according to claim 3 furthercomprising an injection device comprising a prefilled syringe forinjecting liquid, the syringe comprising a tubular body forming areservoir for the liquid, carrying a needle for injecting the liquid,and having a plunger mounted in the body to be movable between a readyposition and an end-of-injection position.
 12. The safety assemblyaccording to claim 4 further comprising an injection device comprising aprefilled syringe for injecting liquid, the syringe comprising a tubularbody forming a reservoir for the liquid, carrying a needle for injectingthe liquid, and having a plunger mounted in the body to be movablebetween a ready position and an end-of-injection position.
 13. Thesafety assembly according to claim 5 further comprising an injectiondevice comprising a prefilled syringe for injecting liquid, the syringecomprising a tubular body forming a reservoir for the liquid, carrying aneedle for injecting the liquid, and having a plunger mounted in thebody to be movable between a ready position and an end-of-injectionposition.
 14. The safety assembly according to claim 6 furthercomprising an injection device comprising a prefilled syringe forinjecting liquid, the syringe comprising a tubular body forming areservoir for the liquid, carrying a needle for injecting the liquid,and having a plunger mounted in the body to be movable between a readyposition and an end-of-injection position.
 15. The safety assemblyaccording to claim 7 further comprising an injection device comprising aprefilled syringe for injecting liquid, the syringe comprising a tubularbody forming a reservoir for the liquid, carrying a needle for injectingthe liquid, and having a plunger mounted in the body to be movablebetween a ready position and an end-of-injection position.
 16. Thesafety assembly according to claim 8 further comprising an injectiondevice comprising a prefilled syringe for injecting liquid, the syringecomprising a tubular body forming a reservoir for the liquid, carrying aneedle for injecting the liquid, and having a plunger mounted in thebody to be movable between a ready position and an end-of-injectionposition.
 17. A safety assembly for a prefilled syringe for injectingliquid, the syringe comprising a tubular body forming a reservoir forthe liquid, carrying a needle for injecting the liquid, and having aplunger mounted in the body to be movable between a ready position andan end-of-injection position, the body of the syringe having a proximalend provided with a flange, the assembly further comprising: a tubularsheath in which the body of the syringe is designed to be housed inaxially displaceable manner between an active position in which theneedle projects through a distal end of the sheath and a protectionposition in which the needle is retracted inside the sheath; resilientreturn means for urging the body towards its protection position;locking means for preventing the body from moving relative to the sheathin the active position by opposing the resilient force of the returnmeans, said locking means being released by release means when theplunger is in its end-of-injection position; the locking meanscomprising two diametrically opposite lugs formed in a wall of thetubular sheath, each lug having a free end provided with a retractablecatch; a cap mounted to slide axially on the sheath between twopositions and having a proximal end with a hole for receiving theplunger; an internal catch for snap-fastening the flange of the tubularbody to the cap, so that due to retraction of the retractable catches,the cap is moved by resilient return means; and two complementaryshoulders acting as abutments, one on the cap, the other one on thetubular sheath, to limit the stroke of the cap once the locking meanshave been released, in opposition to the resilient force of the returnmeans; wherein the sheath and the cap are generally in the form ofbodies of revolution and have complementary means for preventingrelative rotation between each other; and wherein the complementarymeans for preventing relative rotation of the sheath and the capcomprise at least one longitudinal groove formed in the cap andco-operating with a corresponding finger secured to the sheath.
 18. Asafety assembly for a prefilled syringe for injecting liquid, thesyringe comprising a tubular body forming a reservoir for the liquid,carrying a needle for injecting the liquid, and having a plunger mountedin the body to be movable between a ready position and anend-of-injection position, the body of the syringe having a proximal endprovided with a flange, the assembly further comprising: a tubularsheath in which the body of the syringe is designed to be housed inaxially displaceable manner between an active position in which theneedle projects through a distal end of the sheath and a protectionposition in which the needle is retracted inside the sheath; resilientreturn means for urging the body towards its protection position;locking means for preventing the body from moving relative to the sheathin the active position by opposing the resilient force of the returnmeans, said locking means being released by release means when theplunger is in its end-of-injection position; the locking meanscomprising two diametrically opposite lugs formed in a wall of thetubular sheath, each lug having a free end provided with a retractablecatch; a cap mounted to slide axially on the sheath between twopositions and having a proximal end with a hole for receiving theplunger; an internal catch for snap-fastening the flange of the tubularbody to the cap, so that due to retraction of the retractable catches,the cap is moved by resilient return means; and two complementaryshoulders acting as abutments, one on the cap, the other one on thetubular sheath, to limit the stroke of the cap once the locking meanshave been released, in opposition to the resilient force of the returnmeans; wherein the sheath and the cap are generally in the form ofbodies of revolution and have complementary means for preventingrelative rotation between each other; and wherein the complementarymeans for preventing relative rotation of the sheath and the capcomprise at least one axial slot formed in the cap and co-operating witha fin.
 19. The safety assembly according to claim 17 further comprisingan injection device comprising a prefilled syringe for injecting liquid,the syringe comprising a tubular body forming a reservoir for theliquid, carrying a needle for injecting the liquid, and having a plungermounted in the body to be movable between a ready position and anend-of-injection position.
 20. The safety assembly according to claim 18further comprising an injection device comprising a prefilled syringefor injecting liquid, the syringe comprising a tubular body forming areservoir for the liquid, carrying a needle for injecting the liquid,and having a plunger mounted in the body to be movable between a readyposition and an end-of-injection position.
 21. An assembly according toclaim 1, wherein the release means comprises a ramp on the cap.